An engine exhaust system may include various components to enhance engine operation and reduce emissions. These may include selective catalytic reduction (SCR) catalysts, oxidation catalysts, NOx traps, turbochargers, exhaust gas recirculation, etc.
One example of such an engine exhaust system is described in U.S. 2006/0080953. In the exhaust system described in U.S. 2006/0080953, a reducing agent is supplied to an exhaust gas stream upstream of a turbocharger that aids in the breakdown and distribution of the reducing agent (suspended within the exhaust gas stream) prior to the exhaust gas stream reaching a downstream oxidation and SCR catalyst. Furthermore, in this example, multiple separate exhaust gas flows from multiple cylinders are funneled through individual oxidation catalysts, each of the individual oxidation catalysts arranged in an individual tube of an exhaust manifold. Thus, as described in U.S. 2006/0080953, separate exhaust flows are ejected by an engine and immediately passed through separate oxidation catalysts. The separate exhaust flows are then combined into a single exhaust gas flow and injected with a liquid reductant prior to reaching a downstream mixer.
The inventors herein have recognized numerous issues with the above approach. In particular, because the exhaust gases are delivered separately from the cylinders to the upstream catalyst 5, the packaging of multiple oxidation catalysts within individual exhaust manifold tubes may increase packaging constraints on other vital engine components. Correspondingly, the ease of manufacture of such an exhaust system may be reduced and the related manufacturing costs may be increased.
In one approach, a system for treating exhaust gases from an engine, the exhaust gases routed from the engine to atmosphere through an exhaust passage, is provided. The system comprises an injector directing a spray of reductant into the exhaust gases; a first flow combining passage that combines exhaust gas from a plurality of cylinders; an exhaust separation passage, downstream of the first combining passage, that separates an exhaust gas flow into a plurality of separate exhaust gas flows; a plurality of oxidation catalysts, each of which receives one of the plurality of separate exhaust gas flows; a second downstream flow combining passage that receives the plurality of separate exhaust gas flows and combines them into a re-combined exhaust gas flow; a turbocharger that receives the re-combined exhaust gas flow; and a selective catalytic reduction catalyst positioned downstream of the turbocharger.
In this way, by first combining and then separating the exhaust gases ejected by the engine prior to injecting a liquid reductant and passing the re-combined exhaust gas flow through the turbocharger, the exhaust treatment system may be more compactly and flexibly packaged and may thus allow for more flexibility in the arrangement and packaging of other vital vehicle components. Correspondingly, the ease and cost of manufacturing such an exhaust treatment system may be reduced. Furthermore, by first combining the exhaust gases ejected by individual cylinders, separating the resulting single exhaust gas flow and then re-combining the exhaust gas flow into a re-combined exhaust gas flow, the geometrical relationship between the plurality of separated exhaust gas flows upon being re-combined by the second downstream flow combining passage may be configured such that a more turbulent re-combined flow may be realized. This increased turbulence within the re-combined flow may increase the breakdown (into ammonia) and distribution of a liquid reductant (within the re-combined exhaust gas flow) injected therein.
By arranging oxidation catalysts upstream of a turbocharger, the oxidation catalysts and SCR catalyst can be located in warmer locations (i.e., closer to the engine) and may thus allow for both the oxidation catalysts and the SCR catalyst to reach light-off temperature more quickly. As such, fewer emissions may be released to atmosphere during the initial “warm-up” phase of the engine. Additionally, this increased thermal efficiency may reduce the need for parasitic rapid warming conventions (that reduce overall fuel economy) that may use fuel for heating purposes.
Another potential advantage of the present disclosure is that, in some embodiments, the impingement of the exhaust gases upon the rotating blades integral and internal to the turbocharger may aid in the breakdown of the injected urea (suspended within the exhaust gases) into ammonia and in the uniformity of distribution of the ammonia droplets suspended within the exhaust gases. Likewise, the SCR washcoat coating the blades of the turbocharger may further enhance the breakdown of urea into ammonia. Thus, the overall efficiency of NOx removal by the SCR catalyst arranged downstream of the turbocharger may be improved.